Aggregation of peptides into amyloid fibrils or other B-sheet structures is usually related to malfunction. This process is often induced by lipid bilayers but is difficult to monitor in the membrane-bound state. Here, we show how aggregation is readily detected by solid state F-19 NMR using a fluorine-labeled amino acid and how the same sterically rigid side chain can be also be employed to prevent aggregation. Selective CF3-phenylglycine labels were incorporated either as the L- or D-enantiomer into the membrane-active model amphiphitic peptide (MAP). Oriented circular dichroism showed that the D-epimeric peptides maintained an alpha-helical conformation in DMPC at all. peptide-to-lipid ratios, white the L-epimers turned into B-sheet structures above 1:200, just like the wild-type. Aggregation was clearly revealed by the appearance of powder lineshapes in the F-19 NMR spectra of oriented samples. The F-19 NMR dipolar couplings from four D-epimers were analyzed as orientational constraints, showing that the helix of MAP undergoes a concentration-dependent realignment in DMPC from a surface-bound to a tilted state.